The near-far problem is always a problem in CDMA (Code Division Multiple Access) system. If mobile stations transmit at the same power, the radio wave from mobile stations far from a base station is weak at the base station and the radio wave from mobile stations near the base station is strong at the base station, which causes the problem. This has not been a problem in the Frequency Division Multiple Access (hereafter described as FDMA) and the Time Division Multiple Access (hereafter described as TDMA) being conventional multiplexing communication systems.
In FDMA, each mobile station uses a different frequency. Therefore, even if strength of the radio wave from a far mobile station and strength of the radio wave from a close mobile station are different, the base station can distinguish them according to frequencies.
In TDMA, each mobile station uses a different time slot. Therefore, the radio wave from a far mobile station and the radio wave from a close mobile station do not simultaneously come to the base station. Accordingly, even if strength of the radio wave from the distant mobile station and strength of the radio wave from the close mobile station are different, the base station can distinguish them according to time slots.
On the other hand, in CDMA system, the base station simultaneously communicates with a plurality of mobile stations using the same frequency and different codes. Then, if the radio wave from a distant mobile station and the radio wave from a close mobile station come simultaneously at the same frequency, the radio wave from the distant mobile station will be drowned out by the radio wave of the close mobile station. This is an outline of the problem called the near-far problem.
To avoid the near-far problem, the following two methods are currently known. One is the method of using an artificial satellite as a base station and stations on the earth (the airspace also included) as mobile stations. In this case, as to all the mobile stations, the ratio of the distance between the mobile station and the base station is approximately constant. Therefore, even when CDMA system is used for communication, the near-far problem does not occur. This method is used in GPS (Global Positioning System).
The other is the method in which mobile stations control their power by performing communication between the mobile stations and the base station and configuring feedback loops so that the field strength from each mobile station at the base station may become approximately constant. This is called power control. As to the power control, there are two type methods; open loop control and closed loop control. These methods are used by a cellular system such as a cellular phone, and are patents of Qualcomm Incorporated (in USA) (for example, refer to the Patent Document 1, the Patent Document 2, and the Patent Document 3).    Patent Document 1: International Publication WO 91/07037    Patent Document 2: International Publication WO 92/21196    Patent Document 3: International Publication WO 94/19876